1. Field of the Invention
The present invention generally relates to reliable communications systems and, more particularly, to a spread spectrum communications receiver of the direct sequence digital type incorporating improved initial synchronization and tracking circuits.
2. Description of the Prior Art
Spread spectrum communications offer several advantages in communications applications requiring high reliability. These include low density power spectra and interference rejection. In the case of interference rejection, the interference may be accidental, that is, simply a part of the environment of the communication system, or intended, that is, some form of jamming.
There are several types of spread spectrum systems including direct sequence digital systems, frequency hopping systems, time hopping systems, pulsed frequency modulated (or chirp) systems, and various hybrids. Of these, the direct sequence digital systems and the frequency hopping systems are perhaps the more widely implemented. In a direct sequence digital system, a pseudorandom code generator is used to modulate a frequency modulated carrier. In a frequency hopping system, a coherent local oscillator is made to jump from one frequency to another.
The subject invention belongs to the direct sequence digital type of spread spectrum communications system. A description of this and other types of spread spectrum communications systems may be found, for example, in Spread Spectrum Systems, 2nd Ed., by Robert C. Dixon, John Wiley & Sons (1984), and Spread Spectrum Communications, Vol. II, by M. K. Simon et al., Computer Science Press (1985).
Direct sequence digital spread spectrum receivers present a significant design challenge in synchronizing the receiver clock with the transmitter clock, particularly under severe multipath and interference conditions. The sliding correlator technique of acquiring initial synchronization known in the prior art is typically used due to its inherently simple, yet predictable nature. Once synchronization is acquired, a Tau-dither technique known in the prior art is typically used to track the transmitter clock. However, multipath interference tends to result in rapid changing of perceived transmitter code clock rate, often resulting in a loss of synchronization. Attempts to improve the performance of these techniques have been quite complex and expensive.